1. Field of the Invention
This invention relates to systems for the protection of structures and persons from the damaging effects of lightning strikes that originate in clouds. Such lightning strikes include lightning that strikes the ground, or that strikes structures in electrical contact with the ground, or that strikes sensitive apparatus located near the ground.
2. Description of the Related Art
Lightning strikes cause enormous property damage around the world each year and take countless lives, estimated in the thousands. Approximately 44,000 thunderstorms develop on earth every day, generating on average approximately 100 strokes of lightning per second world-wide. Around 100 people in the U.S. are killed from lightning each year, and many more are injured. Lightning causes about 10,000 forest fires in the U.S. per year. It also causes 40% of all farm fires. A review of many years' worth of weather data shows that, in a typical year, lightning is the greatest source of weather damage in the United States, and lightning strike damage amounts to approximately 5% of all insurance claims. National U.S. statistics show that males in their teens and twenties are the most likely to be killed by lightning. Most lightning deaths occur in open fields, near or under trees, or around water. Eighty percent of fatalities occur between the hours of 10 AM and 7 PM. Fifty-two percent of fatalities occur on golf courses, playing fields, on tractors and the like, and 38% occur within a home or structure through contact with a bathtub, faucet, telephone, or appliance. The destruction of at least one jetliner, and one Atlas launch vehicle in March 1987, has been attributed to lightning.
A flash or stroke of lightning releases huge amounts of electrical energy in a very short period of time, less than one second. In particular, the duration of one stroke of lightning is about one fifth of a second. Voltages of the released energy may exceed 100,000,000 volts and peak current may exceed 300,000 amperes. Total energy released in a lightning strike can exceed 40 Gigajoules, enough electrical energy to light an entire city. Temperatures within the lightning stroke, which may range from the width of a pencil to as much as a foot across, can be as high as 50,000 degrees Celsius, five times hotter than the surface of the sun. Lightning flashes or strokes generally range from a hundred meters in length to over eight kilometers in length.
Study has shown that lightning is an effect caused by the stratification of charges within a thunderstorm, producing intense electric fields. Generally, in the case of lightning that strikes the ground (the most hazardous kind), the cloud bottom carries a negative charge. Positive charges may collect on the ground, on buildings, boat masts, people, flagpoles, mountaintops-or trees, in response to the electric fields above. Preceding a lightning flash, a stepped leader-a negative electrical charge made of zig-zagging segments, or steps-extends partway down from the cloud. The steps are invisible; each one is about 150 feet long. When the stepped leader gets within 150 feet of a collection of positive charges, a streamer (surge of positive electricity) rises from the charge collection to meet it. The leader and the streamer form a channel. An electrical current from an object on the ground surges upward through the channel, while negatively charged electrons travel in the opposite direction. This current flow creates a plasma in the atmosphere, generating a bright display called a return stroke. The return stroke is what people see and refer to as a lightning bolt, or flash, or stroke.
Many people feel safe from lightning because of the rarity of lightning in their geographic area, and therefore take no precautions. Nevertheless, even in areas considered to be far from lightning centers, damage can be extensive. A study by Lawrence Livermore National Laboratory, publication number FSIG-95-12-09-1, called "Lightning-Related Occurrences At DOE Facilities", was written in conjunction with development of a guidance document for lightning protection of Department of Energy (DOE) facilities. The study showed that the amount of lightning damage at a facility was not related to the number of regional thunder days reported by the local weather service. This indicates that the effectiveness of the lightning safeguards at a facility are more important than the annual number of thunderstorms in determining the likelihood that lightning caused damage will occur.
It is generally accepted that protecting humans and structures from lightning commenced in earnest with the invention of the lightning rod in 1752 by Benjamin Franklin. Since that time, methods of protection have grown more sophisticated, but have followed the same basic principle of conducting electrical charges around the object to be protected and/or dissipating them into the earth, where they will cause no harm. Protection in the latter half of the twentieth century has become more urgent, however. Some localized assets, such as nuclear power plants, electrical utility towers, rockets, stadia and sports complexes, telecommunications facilities, airports and aircraft, and the like, have extremely high value and are quite vulnerable. At the same time, widespread adoption of silicon transistor circuitry has rendered modem complex control systems more vulnerable to the electromagnetic disturbances caused by lightning.
A great deal of research has been conducted in protecting such high value assets. Some of the current research in the U.S. has been conducted by the National Aeronautics and Space Administration (NASA), for protecting spacecraft and launch facilities, and by the Electric Power Research Institute (EPRI), and in Japan by the Kansai Electric Power Company, for protecting power systems and equipment. The principle technique attempted is to form a conducting path from the ground to a point high in the atmosphere to induce a lightning strike. It is known that lightning is most likely to strike the highest object electrically connected to the ground. NASA conducted research on lightning for many years by firing small wire-trailing, solid-fueled rockets into the lower atmosphere, or by tethering balloons, to induce lightning strikes. Experiments have been conducted at the University of New Mexico, by Philips Labs. These experiments have produced improved prediction methods and better devices for the protection of equipment from lightning strikes.
The majority of currently available lightning protection systems take the form of variations of grounded lightning rods placed atop facilities requiring strike protection. The points of these rods take many forms, in an attempt to dissipate charge without inducing a lightning strike or by producing a conducting path that will induce a lightning strike to hit a protected tower or rod, rather than the facility. The points of such rods are typically manufactured in many fanciful forms, often having names like "Early Streamer Emission Air Terminals" or "Dissipating Arrays". Generally speaking, the effectiveness of these manifold forms is uncertain, at best.
The bulk of the lightning protection research has concentrated upon small refinements to the basic principle of the lightning rod, with refinements to protect equipment from voltage surges. For example, some methods are directed particularly at averting damage to the insulators on power transmission towers. Other efforts are directed to reducing the surge that accompanies a lightning discharge. Still other efforts relate to semi-conducting rods for preventing or diffusing a lightning stroke to mitigate the effects of the electrical surge. Other efforts are directed to preventing a return stroke by using a threshold coupling of a rod to ground.
Because of the huge damage threat to certain major facilities, systems for protecting moderately large sites are frequently sought. To protect larger areas, conducting paths must grow in height to the point that towers are not practical. Wires lofted by rockets, balloons, or kites have been utilized, but mostly for research purposes and not for practical deployment. Because of the cost and danger of using rockets or balloons to induce lightning strikes, recent research efforts have centered on using high-power lasers to create a long-distance plasma or ionization channel through the atmosphere and thereby induce a lightning strike. Research is being conducted at the University of New Mexico by researchers Bernstein, R., Diels, J. C., Stahlkopf, K. E. and Zhao, X. M. using paired UV and visible laser beams. Osaka University and the Kansai Electric Power Company have reported the successful triggering of lightning using a CO2 laser. In all of these methods, the resulting apparatus has been extremely bulky and expensive.
From the discussion above, it should be apparent that there is a need for a system and method for diverting lightning stokes away from valuable assets and harmlessly into the ground. The present invention satisfies this need.